Instantaneous Elements; Instantaneous Element Error During L90 Synchronization - GE L90 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS
The L90 ensures stability in such a case by removing the zero-sequence current from the phase currents prior to
calculating the operating and restraining signals (
is "Enabled"). Removing the zero-sequence component from the phase currents can cause the L90 to
CURRENT REMOVAL
overtrip healthy phases on internal ground fault. This is not a limitation, as the single-pole tripping is not recommended for
lines with tapped transformers.

8.8 Instantaneous elements

8.8.1 Instantaneous element error during L90 synchronization

Two or three L90 relays are synchronized to each other and to system frequency to provide digital differential protection
and accurate measurements for other protection and control functions. When an L90 system starts up, the relays adjust
their frequency aggressively to bring all relays into synchronization with the system quickly. The tracking frequency can
differ from nominal (or system frequency) by a few hertz, especially during the first second of synchronization. The 87L
function is blocked during synchronization; therefore, the difference between system frequency and relay sampling
frequency does not affect 87L function. However, instantaneous elements have additional error caused by the sensitivity of
Fourier phasor estimation to the difference between signal frequency and tracking frequency.
To secure instantaneous element operation, it is recommended either to use FlexLogic as shown here to block the
instantaneous elements during synchronization, or to use a different setting group with more conservative pickup for this
brief interval.
Figure 8-8: FlexLogic to block instantaneous element during 87L startup
The elements must be treated selectively. If, for example, the phase undervoltage setting includes margin sufficient to
accommodate the maximum additional error on startup, blocking or delay are not needed for phase undervoltage.
Similarly, if the phase instantaneous overcurrent setting has sufficient margin, blocking is not needed. Note that significant
zero-sequence and negative-sequence current or voltage error do not appear during L90 startup, therefore all elements
using these quantities are safe.
The table indicates the maximum error and recommended block durations for different elements.
Table 8-1: Maximum error and recommended block duration
Element
Phase undervoltage
Phase instantaneous overcurrent
Ground distance zone 1
Phase distance zone 1
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
SETTINGS  SYSTEM SETUP  L90 POWER SYSTEM  ZERO-SEQ
Maximum error on startup (operate
signal vs. setting)
18%
9%
7%
4%
INSTANTANEOUS ELEMENTS
Recommended block duration
0.7 seconds
0.5 seconds
Not needed
Not needed
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